The Adolescent(1965)

Original title: アドレッセント

A 13-year-old girl, just beginning to realize her own sexuality, comes home one day and finds her mother having sex with a stranger. Horrified, she runs away from home and heads towards her aunt's house in the country, only to be sexually assaulted along the way by a brutish local man. When she arrives at her aunt's house, she discovers that her aunt and cousin are not quite what she thought they were.